1. Field of the Invention
The present invention relates to a pyrolytic boron nitride container. More particularly, the present invention relates to a pyrolytic boron nitride container suitable for retaining a material serving as a source of molecular beams used in molecular beam epitaxy (hereinafter abbreviated as "MBE").
2. Description of the Related Art
MBE is one method of manufacturing thin film, in which a thin-film growth chamber is maintained at ultra-low vacuum of 10.sup.-9 -10.sup.-11 Torr; a container that contains a material serving as a molecular beam source is heated to, for example, a temperature of 500-1600.degree. C.; and molecular beams generated from the melt material are caused to impinge onto a heated substrate, so that a layer having a thickness corresponding to a few atoms is formed on the substrate in a controlled manner. Especially, the MBE method has been widely used for manufacture of epitaxial film of compound semiconductors such as GaAs, and from the viewpoints of purity, heat resistance, and strength, a pyrolytic boron nitride (PBN) container made through chemical vapor deposition (hereinafter abbreviated as "CVD") has been widely used as a container for accommodating a material serving as the molecular beam source.
Conventionally, when an operation according to such an MBE method is carried out for a prolonged period of time, material melt rises along the inner wall surface due to the capillary phenomenon and leaks out of the container, with the result that the material melt adheres to a heater, other heating members, and members inside a furnace, resulting in corrosion, degradation, and/or breakage of these components and members; and/or short-circuit of the heater. Especially, evaporated and scattered material melt is likely to adhere to the upper portion of the inner wall surface of the, container having a low temperature. With time, such material melt adhering to the upper portion of the container may rise along the inner wall of the container and leak out of the container or drop into the material melt to splash droplets of the material melt.
When the above-described phenomena occur, the life-times (or service-lives) of the above-described components and members decrease, resulting in an increase in cost and unstable operation. In addition, splashed droplets of the material melt may adhere to the substrate, resulting in formation of defects in the epitaxial film.
In order to solve the above-described problems, there has been proposed a pyrolytic boron nitride container in which carbon film having a high absorption coefficient with respect to infrared rays (IR) is applied to the outer or inner surface of the container in order to provide a radiant light absorbing layer (see Japanese Patent Application Laid-Open (kokai) No. 2-204391 and Japanese Utility Model Publication (kokoku) No. 7-2617). When the pyrolytic boron nitride container having the radiant light absorbing layer is heated by a heater, radiant light from the heater is absorbed by the absorbing layer, so that the container is heated efficiently and uniformly. Thus, the upper portion of the container is prevented from becoming excessively cool, so that adhesion of the material metal to the upper portion is suppressed.
Graphite and high-melting-point metal are generally considered to be suitable materials of the radiant light absorbing layer. However, these materials involves a fear that they may splash within the furnace and become mixed into epitaxial film, and a fear that since these materials are electrically conductive, if they come in contact with a heater disposed to surround the container, they may cause a short-circuit.
In order to solve this problem, there has been proposed a method to cover a graphite-made light absorbing layer with pyrolytic boron nitride (see Japanese Patent Application Laid-Open (kokai) Nos. 5-105557 and 4-231459). However, since this method involves a process of forming a composite layer of two different materials, manufacture of a pyrolytic boron nitride container involves much time and high cost. In addition, such a pyrolytic boron nitride container has a drawback that the composite layer tends to peel off during use.